1. Type the combat rules down, e.g. what can melee or ranged hit, how do they move etc.
2. Create a rough outline of combat, i.e. how is it executed, step by step.
3. Look at both 1&2 and you should get an idea how to implement the combat system.
4. Once you have a general idea, it is fairly simple to convert rules into Construct events.
So, in short, you have to plan it out. You have to define the combat procedure and rules, just "Three melee, three ranged and three support duke it out with the enemy and uber bosses" is not enough. Sit down with a pen, think on it hard - start with first, simple steps - and note down anything that comes to your mind.
- how many combatants are there?
- how does the combat execute? In turns? One by one? Simultaneously? In real time with 'action points'?
And so on.